If someone wants to craft, for example, an iron trap door, you have to place 4 iron ingots in a square shape. You can do this by single clicking every corner of the square, but also "draw" this recipe by holding down left or right click while moving the mouse.

Now, in snapshot 16w44a, you can only do this, if you have at least one more item than necessary. When you hold only 4 ingots, then draw the square, and release your button, the drawing gets cancelled. This behaviour applies to all recipes / materials and applies to the crafting table (3x3) and also the small crafting grid in the inventory (2x2).

[EDIT] Video for demonstation purpose:
https://youtu.be/48qPDv1iiN4